Properties, Plant and Equipment - Other Property, Plant and Equipment - Tabular Information (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Other Property, Plant and Equipment
Gross other property, plant and equipment	$ 8,466	$ 8,254
Less: Accumulated depreciation and amortization	(5,901)	(5,258)
Net other property, plant and equipment	2,565	2,996
Leasehold improvements
Other Property, Plant and Equipment
Gross other property, plant and equipment	1,186	1,213
Furniture, fixtures, computers and software
Other Property, Plant and Equipment
Gross other property, plant and equipment	4,391	4,170
Vehicles
Other Property, Plant and Equipment
Gross other property, plant and equipment	1,768	1,677
Aircraft
Other Property, Plant and Equipment
Gross other property, plant and equipment	910	910
Other
Other Property, Plant and Equipment
Gross other property, plant and equipment	$ 211	$ 284